Study of the dissociative recombination of HCNH$^+$ and its isomers

The hydrogen isocyanide HNC is less stable than the hydrogen cyanide HCN so that there should be no abundance of HNC compared with HCN in a thermochemically equilibrated system at temperature around 100K. Surprisingly, astrophysical observations of the interstellar medium have reported rather different results for the ratio [HNC]/[HCN]. Therefore, much interest has been recently directed towards the main mechanism of production of these neutral elements, namely the reaction of dissociative recombination (DR) e$^-+$HCNH$^+\rightarrow$ HNC/HCN+H. There exist controversies in the literature on whether the DR reaction proceeds via a direct or an indirect mechanism. Our previous results indicate that the direct DR cross section is small. Therefore, we investigate the indirect mechanism for HCNH$^+$ by electron capture in excited Rydberg states. First, we use a simplified model considering the electronic capture via Renner-Teller non-adiabatic couplings as the decisive step of the DR reaction. This procedure, which already provided good results for other polyatomic ions as H$_3^+$, HCO$^+$ or H$_3$O$^+$, allows an estimation of the absolute cross section. Other possible DR reactions yielding HNC or HCN involve the metastable isomers H$_2$CN$^+$ and H$_2$NC$^+$. We also performed scattering calculations on these systems to estimate the direct DR cross sections.